NotCookie, if I understand you correctly and by your logic, are you suggesting we remove all the lower weighted scores of the Top Performances if you only consider something like the top 5 to represent your personal skills? This would then remove the ability to farm and your personal skill would be totally dependent on your top 5.
Since that is not the case and ALL of our scores are taken into consideration in calculating your total PP and are weighted accordingly, I think the creators of this system intended so that playing "easier songs" to improve the lower weighted scores of your Top Performance would not count as farming and that it is in fact just you trying to improve your skills.
Because of how this new system works by weighting each score (.95^n), I believe even playing a super easy song to improve your 25th highest score of your Top Performances would count as you just trying to improve yourself and is not farming since that score would be weighted .95^25.
But if we go by your logic, then the next logical step would be to completely disregard all the lower weighted scores to remove "farming" which is something I think no one would agree with since having many songs to sum up to your total PP prevents "flukes" and is a much more sound way of ranking players.
Edit: Also the way you define farming implies everyone is farming i.e. you are trying to beat your highest Top Performance by playing a song that is more difficult than your highest Top Performance but mess up in such a way that it still beats your 25th Top Performance and pushes it down increasing your total PP. Boom. Farming.
Edit2: I thought about it a little more. It seems you define someone who is farming by having scores of something like (for the sake of simplicity and ranking from highest to lowest): 8, 8, 8, 8, 8, 8, 8,... and someone who is not farming to have scores of something like 8, 7, 7, 5, 4, 1,..... Then aren't you punishing consistent players by calling what they are doing farming? I believe a player that has shown consistency deserves to be ranked higher than one that has shown to be less consistent.